MSCP SCSI controller speed

9000 VAX vax9000 at
Tue Nov 8 23:23:01 CST 2005

On 11/8/05, Allison <ajp166 at> wrote:
> Considering the major Qbus SCSI controllers do not have near the speed of a
> slow 386 cpu, speed is not the issue. I'd have to look but I think none used
> anything faster than maybe a 10mhz 68000.
> While the 53C90 does have large overhead it's also not quite the bottleneck.

You are right. Those two are not the bottleneck. I measured the actual
execution time when run "dd if=syssrc.tar of=/dev/null" (photo
attached). NetBSD 1.5.2 moves 16kB a time. The measured time is:

4.75ms MSCP-pre-DMA overhead
0.4ms SCSI overhead
7ms DMA of 16kB
4.2ms MSCP-post-DMA overhead
31ms idle time

What kills the speed is the 31ms idle time. I blame NetBSD for that.
The calculated speed is
16kB/(4.75+0.4+7+4.2+31)ms=338kB/s, which is the same as reported by
the "dd" command (267s to move 80.9MB). If there is no idle time, the
speed would be 16kB/(4.75+0.4+4.2)=978kB/s. It beats CQD220.

Thats how much the current design can do. Since it's 2/3 of the
fastest CQD440, I am not going to change the design. I need to test
later with the DEC OS to conform this theory though.

> I would look first at the code.  First place I'd peek at is to see what the
> code overhead is.  Also Qbus transfers can be block mode DMA for best speed
> but watch the length as memory says 4words per block were a maximum to avoid
> bus timout errors.  ISA does not have this problem but it's fairly slow for
> 8bit moves (1meg Byte/S).
> As to beating a RQDX thats easy consider that the MFM drives are 5mbits/S
> range(.625k bytes/S burst) rate and slow seeking combined with no local
> caching more than a sector or two. Also the CPU was a T-11 at a mere 7.5mhz.
> I've considered hacking a SCSI controller for Qbus once.  The approach
> on paper was a PCI PC system board with a Adaptec2900 series controller
> with a parallel adaptor to Qbus.  That was focused on building as little
> hardware as possible becuase the real task was software.  The item
> worked out were software as in  PC level code that didn't require a
> bootable OS (rom resident would be a winner).   However the Qbus side
> was sorted down to a M7941 parallel line unit (PIO) or M7950 (DMA) with
> a driver based on one of the removable non-MSCP cartridge disks (RK, RL
> or RM).  I quit after realizing that I didn't have a 290x SCSI board
> nor wished to program PCs. That and someone gave me a CMD SCSI adaptor. ;)

You are lucky. Nobody gives me CMD card. But from now on I probably
will not need one any more.

vax, 9000

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